The present invention relates to a method and apparatus for measuring the degree of deterioration of a dry developing substance in an electrostatic copying machine or other electrostatographic apparatus.
In an electrostatic copying machine a photoconductive drum is uniformly electrostatically charged and then radiated with a light image of an original document to form an electrostatic image of the document on the drum through localized photoconduction. A developing substance is applied to the drum to develop the electrostatic image into a toner image which is transferred and fixed to a copy sheet to provide a hard copy of the original document.
The developing substance comprises ferromagnetic carrier particles which are magnetically adhered to a rotating cylinder to form a magnetic brush which brushingly engages the drum. Colored toner particles are included in the developing substance which adhere to the carrier particles due to electrostatic attraction caused by dry friction. The copying machine is arranged in such a manner that the electrostatic charge on the toner particles is opposite in polarity to the charge on the drum. Thus, the toner particles are attracted and adhere to the areas of high electrostatic potential of the electrostatic image on the drum which correspond to dark areas of the original document.
The development process depends on the ability of the toner particles to be attracted and adhered to the drum from the magnetic brush. In addition, the toner consumed in the developing process must be replenished to maintain the toner density constant. The toner density is defined as the proportion of toner to carrier particles in the developing substance. However, after the developing substance is used for a considerable period of time the toner particles are attracted to the carrier particles with such a great force that they cannot be readily separated therefrom in the development process. The developing substance under these conditions is said to be deteriorated, or "spent", and results in progressively lower copy density even if the toner density is maintained constant or increased above the normal value.
Thus, in order to provide proper development in electrostatography, it is necessary both to replenish the toner consumed by the development process and sense when the developing substance has deteriorated to the extent that it must be replaced.
It is known in the art to sense the toner density by means of an electromagnetic coil which is electromagnetically coupled with the developing substance. The carrier particles in the developing substance effectively constitute a ferromagnetic core of the coil. The effective inductance of the coil depends on the ratio of toner to carrier particles in the developing substance. The toner density is thereby a function of the coil inductance and may be measured by measuring the impedance to flow of alternating current or the like through the coil. In an improved toner density sensing apparatus, the coil is connected in parallel with a capacitor of known value to constitute a parallel resonant circuit which determines the frequency of an oscillator. The oscillator frequency is thereby a predetermined function of the toner density, and constitutes a measure of the same.
However, the toner density sensing apparatus described is not able to measure the degree of deterioration of the developing substance. A prior art apparatus for measuring said deterioration has been proposed which comprises means for charging a portion of the photoconductive drum to a predetermined potential, developing said portion and measuring the optical density thereof by means of a photoelectric sensor. The lower the sensed optical density, the greater the degree of deterioration.
This optical sensor does not, however, provide accurate measurement of the developer deterioration in practical application due to the numerous variables involved in the copying process. Variations in the output of the charging unit, fatigue of the photoconductive drum, ambient temperature and humidity and other factors influence the results of the measurement to an unacceptable extent.